Recently, the number of people suffering from various circulatory system diseases, including heart infarction and brain infarction, has been on the rise, thus making it more and more urgent to prevent and treat these diseases.
The pathopoiesis of heart or brain infarction is closely correlated to arterial sclerosis. More specifically, if an atheroma is created on the arterial wall or if no arterial cells are produced anymore due to various factors such as elevated blood pressure, then the artery loses its elasticity to become hard and fragile. Also, if the blood vessel is clogged up where the atheroma has been created or if a vascular tissue covering the atheroma has ruptured, then the atheroma will move itself into the blood vessel to clog up the artery elsewhere or to rupture the hardened portions of the artery. As a result, these diseases are caused. That is why it is important to diagnose the arterial sclerosis as early as possible to prevent or treat these diseases.
In the prior art, the lesion of arterial sclerosis is diagnosed by directly observing the inside of the blood vessel with a vascular catheter. However, this diagnosis needs to be carried out with a vascular catheter inserted into the blood vessel of a subject, thus imposing a heavy load on him or her. A known method of diagnosing the arterial sclerosis easily without imposing excessively heavy load on a subject is measuring the index of cholesterol, which is one of major causes of arterial sclerosis, or the blood pressure. However, neither of these values directly indicates the degree of advancement of arterial sclerosis.
Also, if the arterial sclerosis can be diagnosed early enough to administer some medicine to its subject, then the disease can be treated effectively. However, it is said that once the arterial sclerosis has advanced to a certain degree, the farther advancement of that disease can be checked with the administration of medicine but it is difficult to repair the hardened artery completely.
That is why a method or apparatus for diagnosing the arterial sclerosis at an early stage of that disease (i.e., before the disease has advanced to a certain degree) without imposing too much load on a subject is now in high demand.
To meet such a demand, a diagnostic apparatus for diagnosing the arterial sclerosis at an early stage of that disease (i.e., before the disease has advanced too far) without imposing too much load on a subject has been proposed (see Patent Document No. 1, for example). An ultrasonic diagnostic apparatus is a noninvasive medical apparatus that imposes only a light load on a subject. The ultrasonic diagnostic apparatus is superior to an X-ray diagnostic apparatus, which is also a non-invasive medical apparatus, because the ultrasonic diagnosis can be made without administering a contrast medium to the subject or concerning about potential X-ray exposure. Specifically, by irradiating the subject with an ultrasonic wave that has been produced externally, the conventional ultrasonic diagnostic apparatus can acquire shape information or information about the variation in the shape of his or her internal body with time without causing pain to him or her. Thus, the vascular elastic property of the organism can be known and the degree of advancement of the arterial sclerosis can be detected directly.
The ultrasonic diagnostic apparatus disclosed in Patent Document No. 1 can track the object of measurement highly precisely by determining the instantaneous location of the subject by both the amplitude and phase of a detected output signal. According to this technique, vibration components can be measured precisely, and therefore, the thickness variation or strain of the vascular wall can be measured highly precisely on the order of several micrometers.
By adopting such a high-precision measuring technique, the two-dimensional distribution of the elastic property of the arterial wall can be plotted in detail. For example, Non-Patent Document No. 1 shows an example of presenting the two-dimensional distribution of the elastic property in color tones representing the degrees of elasticity by measuring the elastic property of the arterial vascular wall by the method of Patent Document No. 1.
Also, according to Non-Patent Document No. 1, after the elastic property of the iliac bone artery has been measured, the iliac bone is colored in vitro, thereby identifying the respective tissues of the iliac bone arterial wall. And by taking measurements based on the location information of the tissues that have been identified by coloring, it is determined which tissue each elastic property belongs to in the resultant two-dimensional distribution. Non-Patent Document No. 1 also teaches analyzing the defined elastic property of the tissue with a histogram and presuming the type of the tissue by reference to the two-dimensional distribution of the elastic property of the arterial wall, thereby defining the elastic property of each tissue.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226        Non-Patent Document No. 1: Hiroshi Kanai et al., “Elasticity Imaging of Atheroma with Transcutaneous Ultrasound Preliminary Study”, Circulation, Vol. 107, pp. 3018-3021, 2003        